On Tue, 13 Feb 2001 12:57:02 -0600, "Richard Smedley" wrote:

>I assume that your term "speed" and mine "light-gathering power" are
>essentially the same. For telescopes, it is a fact that speed increases
>with diameter, whether a refractive lens or a reflective mirror, given the
>same focal length. This is why those crazy astronomers keep trying to make
>larger and larger mirrors to the point that gravity is now a problem
>(distorts the mirror due to the weight). [...]

This is true for camera lenses, too, but just try to
increase the size of the mirror (or camera lens optics
diameter) after the fact...! ;-) If you figure out how,
let us know!!! ;-) As I pointed out before, though, with
other than very long FL lenses, the front element diameter
doesn't tell you a lot about the lens' "light-gatering power"...